Rogers D W
Institute for National Measurement Standards, National Research Council Canada, Ottawa.
Med Phys. 1992 Sep-Oct;19(5):1227-39. doi: 10.1118/1.596921.
A formalism for clinical external beam dosimetry based on use of ion chamber absorbed-dose calibration factors is outlined in the context and notation of the AAPM TG-21 protocol. It is shown that basing clinical dosimetry on absorbed-dose calibration factors ND leads to considerable simplification and reduced uncertainty in dose measurement. In keeping with a protocol which is used in Germany, a quantity kQ is defined which relates an absorbed-dose calibration factor in a beam of quality Q0 to that in a beam of quality Q. For 38 cylindrical ion chambers, two sets of values are presented for ND/NX and Ngas/ND and for kQ for photon beams with beam quality specified by the TPR20(10) ratio. One set is based on TG-21's protocol to allow the new formalism to be used while maintaining equivalence to the TG-21 protocol. To demonstrate the magnitude of the overall error in the TG-21 protocol, the other set uses corrected versions of the TG-21 equations and the more consistent physical data of the IAEA Code of Practice. Comparisons are made to procedures based on air-kerma or exposure calibration factors and it is shown that accuracy and simplicity are gained by avoiding the determination of Ngas from NX. It is also shown that the kQ approach simplifies the use of plastic phantoms in photon beams since kQ values change by less than 0.6% compared to those in water although an overall correction factor of 0.973 is needed to go from absorbed dose in water calibration factors to those in PMMA or polystyrene. Values of kQ calculated using the IAEA Code of Practice are presented but are shown to be anomalous because of the way the effective point of measurement changes for 60Co beams. In photon beams the major difference between the IAEA Code of Practice and the corrected AAPM TG-21 protocol is shown to be the Prepl correction factor. Calculated kQ curves and three parameter equations for them are presented for each wall material and are shown to represent accurately the kQ curve for all ion chambers in this study with a wall of that specified material and a thickness less than 0.25 g/cm2. Values of kQ can be measured using the primary standards for absorbed dose in photon beams.
基于电离室吸收剂量校准因子使用的临床外照射剂量学形式体系，在AAPM TG - 21协议的背景和符号表示下进行了概述。结果表明，基于吸收剂量校准因子ND进行临床剂量学计算可显著简化剂量测量并降低不确定性。与德国使用的协议一致，定义了一个量kQ，它将质量为Q0的射束中的吸收剂量校准因子与质量为Q的射束中的吸收剂量校准因子联系起来。对于38个圆柱形电离室，给出了两组关于ND/NX、Ngas/ND以及质量由TPR20(10)比值指定的光子束的kQ值。一组基于TG - 21协议，以便在保持与TG - 21协议等效性的同时使用新的形式体系。为了说明TG - 21协议中总体误差的大小，另一组使用了TG - 21方程的修正版本以及国际原子能机构《操作规范》中更一致的物理数据。与基于空气比释动能或照射校准因子的程序进行了比较，结果表明，通过避免从NX确定Ngas，可提高准确性和简化程度。还表明，kQ方法简化了光子束中塑料模体的使用，因为与水中的kQ值相比，kQ值的变化小于0.6%，尽管从水校准因子中的吸收剂量转换为PMMA或聚苯乙烯中的吸收剂量需要一个0.973的总体校正因子。给出了使用国际原子能机构《操作规范》计算的kQ值，但由于60Co射束的有效测量点变化方式，这些值显示出异常。在光子束中，国际原子能机构《操作规范》与修正后的AAPM TG - 21协议之间的主要差异显示为Prepl校正因子。针对每种壁材料给出了计算的kQ曲线及其三个参数方程，结果表明这些曲线准确地表示了本研究中所有壁材料为指定材料且厚度小于0.25 g/cm2的电离室的kQ曲线。kQ值可使用光子束吸收剂量的一级标准进行测量。
